percent of their lifetimes at some point on the main sequence. Where they sit depends on their mass, but all stars on the main sequence generate the energy they need by fusing protons into helium nuclei. And that’s what our sun is doing right now, too. It is middle-aged and still on the main sequence. In the top right of the diagram you find red giants, like Betelgeuse, which is at one corner of the constellation Orion. These are aging stars that have used up most of the protons in their cores and are fueling their furnaces by burning other, larger nuclei. They have cooler surfaces because
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